As electrically writable and erasable nonvolatile memory cells, electrically erasable and programmable read only memory (EEPROM) and flash memory have been widely used. In such nonvolatile memory cells, a floating gate electrode or a trap insulating film, which is interposed between insulating films such as oxide films, is provided under the gate electrode of a metal insulator semiconductor field-effect transistor (MISFET). A state of charge stored in the floating gate electrode or the trap insulating film serves as storage information. The trap insulating film is an insulating layer where charge can be stored, for example, a silicon nitride film. Metal-oxide-nitride-oxide-semiconductor (MONOS) transistors are widely used as nonvolatile memory cells.
For example, Japanese Unexamined Patent Application Publication No. 2015-118974 discloses a technique of forming three kinds of transistors having different withstand voltages and MONOS transistors on a semiconductor substrate.
Furthermore, U.S. Pat. No. 8,916,432 discloses a technique in which an insulating film (ONO film) is formed by sequentially stacking an oxide film, a nitride film, and an oxide film over a semiconductor substrate having a MOS transistor region and a nonvolatile memory transistor, and then the ONO film formed on the MOS transistor region is removed.